Multi-scale modelling of flexible and disordered porous materials
Chimie ParisTech / PSL University 11 rue Pierre et Marie Curie 75005 Paris
The scientific literature of the last few years has seen an exponential growth in the number of nanoporous framework materials reported, with thousands of new metal–organic frameworks (MOFs), covalent organic frameworks (COFs) and molecular framework materials. These novel families of materials open up new horizons in practically all branches of engineering, physics, chemistry, biology, and medicine. Nanoporous materials find numerous applications as selective adsorbents and catalysts, substrates for biosensors and drug delivery, membranes and films in various nanotechnologies, which involve fluids adsorbed or confined to nanoscale pores.
Compared with both dense and nanoporous inorganic materials, many framework materials are based on relatively weaker interactions (coordinative bonds, π−π stacking, hydrogen bonds, and so on) and present large numbers of intramolecular degrees of freedom. All molecular assemblies and solids show some degree of flexibility, yet evidence is accumulating that there is a propensity among these framework materials to display large-scale dynamic behavior, which is typically described by the vague term “flexibility”. This appellation covers phenomena that are very diverse both in terms of their microscopic origins and their macroscopic manifestations. This can also allow them to behave as metamaterials, exhibiting properties that are rare or not found in nature: negative thermal expansion, anomalous mechanical properties such as auxeticity or negative linear compressibility, negative adsorption, etc.
Just as there is flexibility in any molecular assembly, there is no such thing as a crystalline solid without defects and disorder. The prevalence of defects in nanoporous frameworks was first assumed to be mainly due to incomplete activation of framework porosity — resulting in guest molecules blocking the nanopores and negatively impacting surface area and adsorption capacity. However, recent years have demonstrated a larger diversity in their nature (for example, linker and inorganic node vacancies, partial metal reduction and dislocations), as well as the fact they do not necessarily need to have adverse effects but can instead give rise to specific functionalities, such as improving adsorption affinity or catalytic activity. This has led to a change in emphasis in the field, away from the challenge of synthesizing more and more new structures, and focusing more on exploring the physical and chemical properties of those already known. Computational chemistry is playing a big role in this exploration,[6,7] with many of its tools being used alongside ex situ and in situ experimental techniques, including quantum chemistry calculations, first principles molecular dynamics, free energy methods, forcefield-based molecular simulations, as well as coarse-grained MD and other mesoscale modelling methods.
Flexibility, defects and disorder are all related to entropy. They arise from the high dimensionality of the intramolecular degrees of freedom of the materials and the many corresponding ‘soft’ modes of low energy. In the same way, defect formation (free) energies, surface terminations and growth mechanisms are interlinked. To study these complex phenomena, both experimentally and theoretically, we need to combine several characterization techniques at various scales to obtain a global picture. Experimentally, this requires the development of in situ, in operando, time-resolved and spatially resolved methods. Computationally, this means the development of multi-scale modelling strategies, going from the local atomistic picture, to the unit cell, to the crystal and its interfaces… and even to the composite multi-phase micrometer-scale hybrid systems, which are often the form under which the framework materials are used in applications and devices.
This workshop will cover all aspects of theoretical and numerical modelling of porous materials, focusing specifically on the challenges arising from the prevalence of flexibility and disorder in porous framework materials. The goal is to bring together the key players from this field, working at different characteristic time and length scales, and to discuss how to bring together these different techniques in to give a global picture of these systems.
The focus will be squarely put on the fundamental understanding of the physical and chemical behavior of the materials and systems through novel computational methodologies — although the perspectives for enhanced functionality and practical applications are never far behind. The aim of the workshop is to give the scientific community an opportunity to address the challenges of the complexity of the behavior of these materials, resulting in multi-scale collaborations and greater cohesion.
We have identified the following key questions, which represent current blocking points of the field:
- What theoretical and computational toolboxes do we need to address large-scale flexibility and structural disorder, both static and dynamic? How do we address issues such as correlated disorder, seen experimentally for example in the UiO-66 family, and spatial heterogeneity of metal cations in multifunctional MOFs that can contain a mixture of up to 10 different metals?
- We are now seeing disordered states of matter in framework materials, with amorphous glasses, gels, and even liquid metal-organic frameworks. How far do state-of-the-art computational approaches allow us to study these systems, and how do we go beyond these limitations in the future?
- We now have a good experience in second-generation force fields for MOFs, based on ab initio reference calculations (such as MOF-FF, BTW-FF, etc.). How good are they outside of equilibrium conditions, for disordered systems and dynamical properties, to describe large-scale effects such as structural transitions? What is missing and what do we want to add to create the third generation of force fields?
- Transformations in MOFs are affected by crystallite size and morphology, which indicates the need to go beyond the approximation of periodic boundary conditions to simulate such processes. The same holds – as it is known from e.g. from semiconductor simulation – for defects and disorder. Approaches to investigate MOF growth, surface and defect free energies of formation by theoretical methods are interlinked and need to be developed.
- There is a need for simulations of systems at the crystal scale or larger (for composite systems), including explicit simulation of interfaces. Between coarsed-grained approaches and finite elements-based numerical modelling, how do we approach these systems? How do we link these simulations with classical atomistic models?
- Toward applications: how can we leverage flexibility and disorder to target desirable physico-chemical properties? Starting points include induced-fit effect, mechanical improvement of amorphous structures over crystalline ones, ability to create functional glass monoliths, etc.
François-Xavier Coudert (CNRS & Chimie ParisTech) - Organiser
Alain Fuchs (CNRS, France) - Organiser
Rochus Schmid (Ruhr-Universität Bochum) - Organiser